This invention relates to thermal barrier coating systems for protecting substrates from high external temperatures, and, more particularly, to such a system wherein a bond coat is not required.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The hotter the exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the exhaust combustion gas temperature. However, the maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the combustion gases impinge when they are at their hottest temperatures. In current engines, the turbine vanes and blades are made of nickel-base superalloys, and can operate at metal temperatures of up to about 2100.degree. F.
Many approaches have been used to increase the operating temperature limit of the turbine blades and vanes. The composition and processing of the materials themselves have been improved. Physical cooling techniques are used. In one approach, internal cooling channels are provided within the components, and cool air is forced through the channels during engine operation. In another approach, a thermal barrier coating system is applied to the turbine blade or turbine vane component, which acts as a substrate. The thermal barrier coating system typically includes a ceramic thermal barrier coating material that insulates the component from the hot exhaust gas, permitting the exhaust gas to be hotter than would otherwise be possible with the particular material and fabrication process of the component, or, alternatively, reducing the amount of cooling air that is required to cool the metallic substrate portion of a coated component. These two approaches, cooling air forced through internal cooling channels and thermal barrier coating systems, have also been combined to allow the operating temperatures to be further increased.
It has been disclosed that aluminide coating life can be improved by reducing sulfur in the substrate prior to the application of the aluminide coating. See Final Report to Navla Air Development Center R90-917552-1. The aluminide coating is less tolerant of sulfur than a superalloy substrate. It has also been reported by Smialek, "The Effect of Sulfur Removal on Al.sub.2 O.sub.3 Scale Adhesion" The Mineral, Metals and Materials Society, 1989, that reducing sulfur in the substrate also improves the adhesion of aluminum oxide scale in NiCrAl alloys, but has marginal effect in improving adhesion of aluminum oxide scale in NiCrAlY alloys because yttrium acts to eliminate the negative effect of sulfur.
Typical thermal barrier coating systems employ ceramic materials which usually do not adhere well directly to the superalloys used in the substrates. Therefore, an additional metallic layer called a bond coat is placed between the substrate and the thermal barrier coating. Bond coats typically are aluminides or MCrAlY's, where M includes nickel. The bond coat composition is selected to adhere to the underlying substrate, to adhere well to the overlying thermal barrier coating, and also to reduce the tendency of the substrate to degrade from oxidation and corrosion. The bond coat resists the oxidizing effects of the hot combustion gas stress in a gas turbine engine, but other elements present in the coating contribute to the ability of the ceramic coating to adhere to the substrate material, as noted in Strandman et al., U.S. Pat. No. 4,880,614. Commonly used bond coat materials include MCrAlY alloys, where M is an element selected form the group consisting of Ni, Co and combinations thereof, and aluminides such as the platinum aluminides (PtAl).
Structures such as thermal barrier coating systems and internal cooling passages permit hot-section gas turbine components to be used at higher temperatures than otherwise possible. However, at the same time steps must be taken to ensure that the components do not degrade from oxidation, hot corrosion, and other effects in the hostile environment of the high-temperature, high-velocity combustion gas. In one approach to improving environmental resistance, the composition of the substrate is modified. For example, yttrium, zirconium and/or rare earth elements are sometimes added to the substrate alloy in small amounts, and these additions have been shown to improve resistance to high-temperature oxidation and corrosion, thereby extending the life of the substrate. Although the mechanism is not fully understood, these additions are particularly beneficial in improving the adhesion of the oxide scales which form on the substrate, acting to protect it.
In another approach, an environmentally resistant coating may be deposited over the substrate as part of a thermal barrier coating systems to protect the substrate against oxidation and hot corrosion damage. The environmentally resistant coating is typically an aluminum-rich alloy whose surface oxidizes to form an aluminum oxide at elevated temperature. The aluminum oxide is present as an adherent scale that acts as a diffusion barrier against diffusion into the substrate of such detrimental substances as oxygen, sulfur, nitrogen, and other potential damaging components of the combustion gas.
These differing approaches to achieving long operating lives at high temperatures are often used in combination, leading to the best results currently obtained with metallic gas turbine blades and vanes. However, as the design and metallurgical techniques have become more sophisticated, in some instances they have also become quite expensive. The many steps required for the application of thermal barrier coating systems raises the price of the coated components. The substrates themselves have also become more costly to melt and cast. For example, when they are used even in relatively small amounts, the presence of additions such as yttrium, zirconium and/or rare earth elements requires that special ceramics be used in the casting process. Also, the more complex the structures and manufacturing processes, the more potential failure modes and possibilities for manufacturing errors.
There is an ongoing need for improved superalloy articles, and methods for their preparation, which attain long operating lives at elevated temperatures, but also have better producibility, lower cost, and less complex structures. The present invention fulfills this need, and further provides related advantages.